Drugs that affects the microtubule dynamics constitute one of the most important classes of chemotherapeutic agents. Anti-microtubule drugs of clinical relevance include paclitaxel (taxol), vinca alkaloids (vinblastin and vincrisitn), nocodazole and colchicine. These agents trigger a checkpoint, the spindle checkpoint, which monitors he attachment of chromosomes to the spindle, and elicits arrest in mitosis generally followed by apoptosis. Several cellular factors which participate in this checkpoint have recently been identified, however, the exact molecular mechanisms through which errors in spindle assembly, or chromosomes attachment to the spindle engage the cell cycle machinery remain to be elucidated. We made the novel finding that a transcription coactivator possessing acetyltransferase activity, p300, enhances the mitotic arrest elicited by taxol. Acetyltransferases belonging to the p300 family have been implicated in conveying adaptive responses in a variety of signal transduction pathways, through regulation of transcription of many cell-cycle regulatory genes. We now demonstrate that p300 associates with mitotic and interphase microtubules, it acetylates tubulin, and it favors tubulin polymerization in a taxol-dependent assay. Moreover p300 levels and its association with microtubules are significantly increased in taxol treated cells. Based on these results we hypothesize that p300 acts as an important effector of sensitivity of tumor cells to taxol, through its association with tubulin and through its activity as a transcription factor. To test this hypothesis we will: l)Identify the regions of p300 responsible for its interaction with microtubules and generate mutants with corrupted tubulin-binding ability (loss or gain of function, respectively). 2) Study how these mutants influence cytoskeleton architecture, spindle assembly and nuclear import of acetylated transcription factors in taxol treated cells. 3)Define the mechanisms by which p300 participates in cell cycle arrest and apoptosis induced by taxol and identify the molecular events occurring downstream of taxol which are influenced by p300. 4)Provide a rationale and a strategy for the design of molecules, such as peptides which mimics p300 effects on apoptosis, able to enhance chemosensitivity to taxol. Since mitotic spindle inhibitors constitute a growing class of anti-cancer agents, it is essential to understand molecular mechanisms of resistance and sensitivity. Thus, studies proposed in this application are expected to have important clinical implications.